An essential problem in radio systems is the rapid variation of the properties of a radio channel as a function of time. This particularly relates to mobile telephone systems, wherein at least one of the parties to the connection is often mobile. This being so, the attenuation and the impulse response of the radio channel vary within a wide variation and amplitude range as many as thousands of times per second. The nature of the phenomenon is random, which enables statistical description thereof mathematically. In addition to an entirely statistical manner, other methods also exist, for instance geometrical methods or combinations of the aforementioned. The phenomena impair the design of radio connections and the devices to be employed.
There are many reasons for variation in a radio channel. During the transmission of a radio-frequency signal from a transmitter to a receiver, the signal propagates in the radio channel along one or more paths, in each of which the phase and amplitude of the signal vary, causing fadings of different duration and strength in the signal. In addition, noise and interference caused by other transmitters also interfere with the radio connection.
A radio channel can be tested either under real circumstances or with a simulator simulating real circumstances. Tests performed under real circumstances are difficult, since tests performed outside, for example, are subject to weather and the time of year, for example, which change all the time. Measurements made in the same place produce different results in different times. In addition, a test performed in one environment (city A) is not completely valid in another corresponding environment (city B). The basic problem is always that the tests are not repeatable, and the effects of the different factors cannot be tested separately. The worst possible situation cannot usually be tested under real circumstances.
The implementation of a real-time simulation requires much effect from the simulation equipment. Real-time simulation means that the simulation time corresponds to real time. Modelling a radio connection between a realistic transmitter and receiver most frequently requires that several signal propagation paths between the transmitter and the receiver are taken into account. In addition, if more than one antenna (MIMO, Multiple Input, Multiple Output) is employed in the transmitter and/or the receiver, the number of propagation paths required continues to increase. One propagation path including a plurality of parallel paths corresponds to one radio channel to be modelled. The number of channels employed in several known simulation devices is eight. If four antennas are employed in both the transmitter and the receiver, 16 channels are required in the simulation.
A special addition to the number of channels is caused by possible sources of interference, which also have to be taken into consideration in a realistic simulation.
Realistic simulations have been impossible to implement as such in any manner because of the complexity of the matter. Real channel models cannot be simulated, since the simulation of a simple MIMO implementation alone requires the use of the most efficient hardware simulators. At present, simulations on the link level have been performed only based on software by the use of very coarse approximations only. For example, the use of real channel models has not been possible in any manner.